Canadian river dries up in four days after glacier retreats

An aerial view of the Kaskawulsh Glacier taken in September (Dan Shugar / University of Washington Tacoma)

A large Canadian river dried up over the course of four days last year after the glacier which fed it retreated at an extraordinary rate.

Over the past century, the Kaskawulsh Glacier in the country's north has retreated around 1.6km, but last year it happened so quickly the water actually changed course.

The Slims River case is the first documented instance of modern-day 'river piracy'.

Also known as 'stream capture', the process has been documented in a new study led by the University of Washington Tacoma and published in Nature Geoscience.

The study provides a stark and disturbing peek into the effects of climate change.

Satellite images of the Kaskawulsh Glacier (bottom left) and the river flow of the Slims River (European Space Agency)

For centuries, Slims River has flowed out to the Bering Sea, with the smaller Kaskawulsh River flowing in another direction to the Gulf of Alaska.

But Slims River has now all but dried up; river gauges documented an abrupt drop between May 26 and 29.

Assistant Professor of Geoscience and lead author Dan Shugar, along with co-authors Jim Best and John Clague, had planned fieldwork for August - but when they arrived, there was little river to see.

"There was barely any flow whatsoever. It was essentially a long, skinny lake.

"The water was somewhat treacherous to approach, because you're walking on these old river sediments that were really goopy and would suck you in. And day by day we could see the water level dropping," Assoc Prof Shugar says.

A 30-metre canyon had been carved through the end of the glacier causing meltwater to flow through one lake into another "almost like when you see champagne poured into glasses that are stacked in a pyramid".

The ice-walled canyon at the toe of the glacier with recently collapsed ice blocks (Jim Best / University of Illinois)

"Geologists have seen river piracy, but nobody to our knowledge has documented it happening in our lifetimes," Assoc Prof Shugar says.

It has been documented "thousands or millions of years ago - not the 21st century where it's happening under our noses," he says.

It normally happens as a result of tectonic motion in the Earth's crust, landslides, erosion and, in this case, changes to glaciers.

The dramatic shift in the landscape will have important downstream effects for wildlife, tourism and communities which live near and depend on the river.

Slims River crosses the Alaska Highway and is popular with hikers and now with the riverbed exposed Dall sheep from the nearby Kluane National Park could wander into territory where they can be legally hunted.

The Kluane Lake, which is normally fed by the Slims River, didn't refill during the Canadia summer, leaving it around 1m lower than previously recorded.

The small communities of Burwash Landing and Destruction Bay are now further from Slims River's shore and as the lake level keeps dropping, it's expected it will become an isolated lake.

Assoc Prof Shugar says it'll mean animals and people who depend on the river will need to adjust to a new reality with changes in sediment transport, lake chemistry, fish populations and wildlife behaviour.

Meanwhile, the Alsek River, which is fed by the Kaskawulsh River, was running higher than previous years. The river is a UNESCO world heritage site and popular for whitewater rafting.

Researchers say there's a 99.5 percent probability the glacier's dramatic retreat is the effect modern day climate change.

"I always point out to climate-change sceptics that Earth's glaciers are becoming markedly smaller, and that can only happen in a warming climate," Dr Clague says.